Find the conversion factor that will allow you to get from the units that you have to the units that you need. For example, to get from inches (in) to centimeters (cm), you will need to know that the conversion factor is 2.54 cm/1 in (read 2.54 centimeters per inch). If you have 5 inches to convert, the problem is set up like this: 5 in/1 x 2.54 cm/1 in. When you multiply, you will cancel the 1 inch into the 5 inches so that the "inches" go away. This leaves 5 x 2.54 cm = 12.7 cm.

Use the same type of unit conversion technique when converting one size measurement into a different type of unit within the same system. To convert milliliters (mL) into liters (L), use the conversion factor 1 L/1,000 mL. 5,000 mL would convert to liters this way: 5,000 mL x 1 L / 1,000 mL. The milliliters and the thousands cancel each other out, leaving 5 x 1 L= 5 L.

Use multiple unit conversion factors when more than one step is required to get from the units that you have to the units that you need. To convert a yard into centimeters, you have to convert yards to inches and inches to centimeters. The problem would set up like this: 1 yd x 36 in/yd x 2.54 cm/in. The yards and inches both cancel out. This leaves: 36 x 2.54 cm = 91.44 cm. The key is to always place the measure that you are getting rid of in a position so it can be canceled during the calculation and leave only the desired units in the answer.